A spinal disc is composed of two main parts, a gel-like inner portion called the nucleus pulposus, and a tough outer portion called the annulus fibrosus. The annulus is similar to a radial tire surrounding the nucleus. In a healthy disc, the nucleus is contained, and for the most part, centered within layers of the annulus, allowing for even distribution of pressures thereby providing cushioning to the adjacent vertebrae surrounding the disc.
An artificial disc (also called a disc replacement, disc prosthesis or spine arthroplasty device) is a device that is implanted into the spine to imitate the functions of a natural disc (carry load and allow motion). There are many artificial disc designs classified into two general types: total disc replacement (TDR) and nucleus disc replacement (NDR). As the names imply, with a TDR, all or most of the disc tissue is removed and a replacement device is implanted into the space between the vertebra. With an NDR, only the center of the disc (the nucleus) is removed and replaced with an implant. The outer part of the disc (the annulus) is not removed. NDR surgery offers certain benefits compared to TDR. Since an NDR device is designed to replace only the nucleus of the disc, the procedure is less time consuming and posses less risk and maintains the functionality of surrounding structures.
One of the primary hurdles an NDR device has to overcome is the ability to stay within the annulus of the disc. In order to closely mimic the native nucleus, most NDR devices are made of soft and pliable plastic-like (biopolymer) materials. One such material is called hydrogel, which expands as it absorbs water. The device is placed into the nucleus cavity of the disc and hydrates to expand and fill the cavity. The device is flexible or compressible and by this means, allows motion, much like a natural disc nucleus. In particular, the device can elastically deform during normal activities (axial loading, flexion, extension). As a result, the device can potentially squeeze out of the opening(s) formed in the annulus and used to introduce the NDR into the annulus. Another issue is that the containment wall of the NDR is often a degenerated annulus that has existing fissures or will develop fissures in which the NDR device can expulse over time.
Accordingly, there remains a need for improved methods and devices for replacing a nucleus of a spinal disc, and in particular for preventing nucleus disc replacement expulsion.